1. Field of the Invention
This invention relates to a discharge protection circuit, and more particularly, to a discharge protection circuit for discharging a full-bridge circuit in a brushless DC fan.
2. Description of the Related Art
The structure of a conventional brushless DC fan as shown in FIG. 1 comprises a fan 10, a control circuit 11, a full-bridge circuit 12 and a discharge protection circuit 13. In the full-bridge circuit 12, a switch L1 is in reverse state of a switch L3, and a switch L2 is in the reverse state of a switch L4. Using the switches L1 and L3 as an example, if a node A1 in the control circuit 11 outputs a high-level signal, a node A3 in the control circuit 11 will output a low-level signal to set the switch L1 in the full-bridge circuit 12 to ON, and the switch L3 in the full-bridge 12 to OFF. On the other hand, if the node A1 in the control circuit 11 outputs a low-level signal, the node A3 in the control circuit 11 will output a high-level signal to set the switch L1 in the full-bridge circuit 12 to OFF, and the switch L3 to ON.
In order to prevent malfunction in switches L3 and L4 in the full-bridge circuit 12, any possible remaining current in the switches L3, L4 is grounded by the discharge protection circuit 13 when the switches are OFF. The conventional discharge protection circuit 13 shown in FIG. 2 comprises two switches S1 and S2, and three resistors R1, R2, and R3. The course of operation is: when the node A1 of the control circuit 11 outputs a low-level signal and the node A3 outputs a high-level signal, the switch S1 is ON, a power Vcc is grounded, and the switch S2 is OFF, thus the switch L3 of the full-bridge circuit 12 operates normally. When the node A1 of the control circuit 11 outputs a high-level signal and the node A3 outputs a low-level signal, the switch S1 is OFF, the power Vcc sets the switch S2 to ON, and thus the switch L3 of the full-bridge 12 is grounded to discharge current to prevent any malfunction.
The foregoing discharge protection circuit uses relatively more electronic elements and the circuit structure is more complicated. Therefore, how to reduce the number of electronic elements used by a discharge protection circuit in order to simplify its design while maintaining the discharge protection function is the focus of the invention.